1. Field of the Invention
The present invention relates to a compound lens and an optical system using the compound lens.
2. Description of the Related Art
A transfer type color image forming apparatus is equipped with multiple image forming units corresponding to separated color components and an optical scanner to provide image data corresponding to color components, that is, multiple laser beams to these image forming units.
The optical scanner has multiple semiconductor laser elements as light sources, a first lens group (a pre-deflection optical system) that narrows multiple laser beams emitted from these semiconductor laser elements to a specified diameter, an optical deflector that continuously reflects the multiple laser beams narrowed by the first lens group in the direction orthogonal to the conveying direction of a recording medium, a second lens group (a post-deflection optical system) that focuses the laser beams deflected by this optical deflector at a specified position of a recording medium and so forth.
Further, in many cases the direction in which laser beams are deflected by the optical deflector is regarded as the main scanning direction and the direction orthogonal to the recording medium conveying direction, that is, the direction orthogonal to the main scanning direction is regarded as a sub-scanning direction.
By the way, for instance, when a glass cylinder lens is used in the pre-deflection optical system and a plastic lens is used in the post-deflection optical system, a color distortion is produced as the refractive index of the plastic lens is changed by temperature and humidity.
So, the color distortion is corrected by providing two sheets of plastic lenses to the focusing lens system of the post-deflection optical system by considering the change in refractive index by temperature and humidity.
However, this correcting method has no problem when the resolution is low but when the resolution is made high, the color distortion must be corrected by the post-deflection lens only and therefore, there was such a problem that a plastic lens becomes in complicated shape and the color distortion in the sub-scanning direction cannot be corrected completely.
So, an optical scanner is considered, that is capable of correcting a color distortion by providing a plastic lens in the pre-deflection optical system.
In his method, however, as two sheets of lenses are arranged at separated positions in the pre-deflection optical system, unless the optical axes of two lenses are in accord with each other, the beams are inclined and as a result, a desired beam shape cannot be obtained on an image surface. Therefore, there was such a trouble that a time is needed for an adjusting work that becomes necessary to bring the optical axes of two lenses in accord with each other.